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Peaceful shellfish changed (4)

Ross

I believe the answer is:

calm

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'peaceful' is the definition.
(associated in meaning)

I don't understand how the rest of the clue works.

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(Other definitions for calm that I've seen before include "Tranquil, undisturbed" , "Peace, tranquillity" , "Peaceful, undisturbed" , "Still and quiet" , "Halcyon" .)

I've seen this clue in the Sydney Morning Herald.
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